Method of precipitation hardening iron alloys



y- 1954 F. M. WALTERS, JR., ETAL 2,683,677

METHOD OF PRECIPITATION HARDENING IRON ALLOYS Filed July 2'7, 1942 MAX'MUM HARDNESS OBTAINED ON HOLDING manness-aocxwsuh AT soo'c. (952 F.)

ALLOY Af AIR FURNACE WATER AIR FURNACE COMPOS'T'ON lrfizimt QUENCHE COOLED coquzo oueucmzn COOLED COOLED Ni l A] ATURE a 26 32 as 42 40 3a 794 055 1.57 634 c 24 22 2e 39 37 36 am an; we see o 22 2o 2: 3s 33 32 .97 0.4: 0.50 336 Il1:. E

LEGEND r x NICKEL v o NlCKEL MANGANESE 1o INCREASED 'HARDNESS nocxweu. c

TRANSFORMATION TEMPERATURE, DEG. C.

Francis M Walters, Jr.

Irvin R. Kramer Patented July 13 1595 4 METHOD OF PRECIPITATION HARDENING IRON ALLOYS Francis M. Walters, .lr., Washington, D. 0., and Irvin R. Kramer, Baltimore, Md.

Application July 27, 1942, Serial No. 452,536

4 Claims.

(Granted under Title 35, U. S. Code (1952),

sec. 256) This invention relates to a class of precipitation hardening low carbon (0.0 to 0.1%) iron alloys which do not require quenching.

Carbonless or low carbon iron alloys of this range may strengthened in several ways. Of these the one that produces the greatest increase in strength is that called precipitation or age hardening.

Precipitation hardening may take place in alloys in which one of the constituents is soluble in the base eta} in greater percentages at high temperatures than at low temperatures. In such alloys the portion of the element non-soluble at low temperatures is precipitated throughout the alloy in a more or less finely divided state. If the particles of this phase (metal or intermetallic co pound) are of a certain critical size and sly distributed the alloy possesses considen. strength and hardness.

"With such alloys th conventional method for producing precipitation hardening has been to 1 from temperature at which the precipitating element goes completely into solution. This quenching treatment makes the alloy soft. It is then heated back to some temperature below the solution temperature and held long enough for sufficient precipitation to occur to give the physical properties desired. The time which an alloy is held at the hardening temperature depends upon its composition and upon the temperature. Cold work before the precipitation treatment increases the rate at which hardening takes place.

Conventional precipitation hardening alloys are subject to certain limitations. The fact that they must be quenched from the solution temperature limits the thickness of material which can be precipitation hardened, since the inside of a piece of metal cools more slowly than the outside. the piece is too thick so much precipitation takes place during cooling that the center is over-aged Another trouble encountered with some precipitation hardening alloys is that precipitation occurs preferentially at the grain boundaries instead of u orrnly throughout the grains. This results in intergranular brittleness.

Several alloying elements are known to produce precipitation hardening in iron. Of these copper is used commercially. While precipitation hardening iron copper alloys do not need to he quenched, the added strength conferred by the copper amount to only twenty thousand pounds per square inch. Molybdenum and tungsten make iron precipitation hardening but either 8 per cent of molybdenum or 16 per cent of tungsten are required. Iron may be precipitation hardened also by titanium, beryllium, or by the formation of intermetallic compounds of such elements as nickel and aluminum. All of these alloying elements except copper produce precipitation hardening by the conventional quenching method.

If precipitation hardening iron alloys are to be useful, either they must have properties not obtainable in iron alloys containing carbon or they must be superior to steels in some other respect. To harden a piece of steel it is necessary to cool it rapidly from a fairly high temperature. This rapid cooling will result in a certain amount of distortion unless troublesome precautions are taken. The depth to which steels harden, even alloy steels, is limited and it is impossible to develop the same strength in a heavy section as lat which may be secured on heat treating a small section.

Precipitation hardening iron alloys would be useful, then, if they could be made hardenable without quenching. If such alloys did not re quire quenching, they would also be hardenable in heavy sections.

It is an object of this invention to provide a class of iron-base low-carbon alloys which are precipitation hardening without quenching.

It is a further object to provide a class or" such alloys which, on reheating to the precipitation temperature, precipitate the alloying element very quickly.

It is another object to provide a class of such alloys having the above good qualities and which are free from grain boundary precipitation and are therefore free from brittleness.

To make a precipitation hardening iron alloy which does not require quenching we combine with a suii'icient percentage of the precipitating alloying element or elements, enough nickel or manganese (or both) to lower the decomposition temperature of austenite to a temperature at which the rate of precipitation is inappreciable.

The principles underlying these new alloys are three:

A. Many of the alloying elements which cause precipitation hardenin in iron are more soiuble in austenite than they are in ferrite.

B. Most precipitation hardening elements do not come out of solution rapidly unless the temperature is above 450 C.

C. Nickel and manganese lower the temperature at which austenite decomposes.

A large number of carbon-free iron alloys have been prepared and their hardness determined after quenching, air cooling and furnace cooling, and after reheating to various temperatures and holding for various lengths of time. These alloys include those in which the austenite decomposition temperature was lowered by nickel, by manganese, or by nickel and manganese together. The hardening elements have included aluminum (with nickel), molybdenum, copper (with silicon or aluminum), titanium, boron and berylium.

In the accompanying drawings Fig. 1 is a table showing the hardness characteristics of a group of alloys under various conditions of treatment, and

Fig. 2 is a graph showing the influence of transformation temperatiu'e on the increase of hardness between water quenching and furnace coolmg.

The effect of the three cooling rates on hardness is shown in Fig. 1, in which alloys A and B have transformation temperatures which are too high to secure the desired result, while in alloy C the manganese content and in alloy D the nickel content has been increased sufficiently to bring the transformation temperature within the range desired for the attainment of the desired qualities. It will be noted that alloys A and B harden on furnace cooling considerably in excess of the amount of their hardening on water quenching. It can also be seen that even on air cooling they are much harder than on quenching. Furthermore, they do not harden appreciably on further treatment.

This is in contrast to the performance of alloys C and D which harden uniformly with all three forms of cooling and undergo marked increase in hardening upon further treatment.

The relationship between the transformation temperature and the tendency to harden on cooling from the austenitic condition is shown in Fig. 2, where the difference in hardness between furnace cooled and quenched samples is plotted against the temperature at which austenite begins to decompose. If this temperature is below 525 C. (980 F.) the alloys show the same hardness whether water quenched or furnace cooled. The samples used in preparing this graph all utilized either nickel or nickel and manganese for the purpose of lowering the transformation temperature, as indicated by the legend. While aluminum was used as the hardening agent, similar results have been obtained using the other precipitation hardening agents listed above.

The time hardness relationship of these precipitation hardening alloys is similar to that of the conventional precipitation hardening alloys. The two typical alloys shown in Fig. 1 as C and D attain a higher maximum hardness at lower temperatures than that reached at the higher temperatures, although the time required is longer. At 550 C. the alloys over-age rapidly and show a decrease in hardness after two hours. However, at temperatures below 500 C. the alloys maintain their hardness after 48 hours at temperature.

The effect of heat treatment on the mechanical properties of the furnace-cooled alloys of the invention is similar to that of the usual precipitation hardening alloys. The tensile and yield strength increase with time at temperature until maximum values are reached corresponding to the flat portion of the time hardness curves. However, the elongation and reduction in area do not change with tensile strength in exactly the same manner as steels. Thus they may decrease when the alloys are held at a temperature for a long time even though the tensile strength does not change. This decrease in ductility depends upon the composition of the alloy and the temperature employed. Allow D containing 0.5 per cent of aluminum maintains its ductility and strength regardless of exposure to temperatures below 500 C. At a temperature of 550 C. the alloy over-ages rapidly so that upon holding for 48 hours the tensile and yield strengths decrease below that of the original furnace-cooled values although the ductility is still very good. In alloy C in which the aluminum content (1.86%) is much higher, the ductility is good, but the alloy is more susceptible to the heat treating procedure. When this alloy is exposed for long periods of time at temperatures of 500 C. or 450 C. the ductility decreases. This is shown by a comparison of the properties after holding for 4, 8 and 48 hours at 500 C. At the end of four hours the properties are good and compare very favorably with those of quenched and tempered SAE steels of the same tensile strength. However, when held for 8 hours the ductility de creases and at 48 hours the alloy is extremely brittle. When the alloy is completely over-aged by holding at 550 C. for 48 hours, the tensile strength is lowered and the ductility fully recovered.

The class of alloys possesses a good ductility in respect to both elongation and reduction in area for curves falling above the cure for SAE steels. The ductility of these alloys as measured by the impact-resistance is good. Nickel steels are known for their high impact values and the alloys of this class exceed even these values.

Freedom from grain boundary precipitation in these alloys is very good. For example, a photomicrograph of alloy D after furnace cooling from the solution temperature and holding for l hours at 500 C. failed to reveal any grain boundary precipitation even at 1500 magnification. These alloys possess many outstanding advantages. Since they may be hardened without quenching, heavy sections in structures dimcult to heattreat because of shape may be heat-treated. They are especially adaptable to heavy armor. Since they do not have to be quenched, uniform properties may be developed throughout thicknesses as great as 24 inches. Although the alloy content of these alloys is somewhat higher than that of the usual armor compositions the cost for alloy elements is more than offest by the lower cost of heat treatment. Furthermore, part of the nickel content may be replaced by manganese. The fact that they may be machined while soft and then hardened with little or no distortion should greatly aid in manufacturing processes.

While only two alloys, those marked C and D in Fig. 1, have been specifically set forth, it should be distinctly understood that these particular alloys were used merely by way of example and that the invention extends beyond them to a large class of similar alloys possessing the characteristics set forth in the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

We claim:

1. A method of precipitation-hardening a low carbon iron base alloy containing a minor proportion of a precipitation-hardening constituent and of a constituent from the group consisting of nickel and manganese, said alloy being capable of undergoing transformation from austenite to ferrite on cooling and having a constituent content from said nickel and manganese group which is sufficient to lower the austenite-ferrite transformation temperature of the alloy to a level at which the rate of precipitation on air cooling is inappreciable, which comprises slow cooling said alloy at a rate not substantially greater than that of air cooling from austenite solution temperature through said transformation temperature, holding the alloy at a precipitation-hardening temperature for a time sufiicient to develop hardness therein, and cooling the alloy.

2. A method of precipitation-hardening a low carbon iron base alloy containing a minor proportion of aluminum as a precipitation-hardening constituent and of a constituent of the group consisting of nickel and manganese, said alloy being capable of undergoing transformation from austenite to ferrite on cooling and having a constituent content from the group consisting of nickel and manganese which is sufficient to lower the austenite-ferrite transformation temperature of the alloy to a level at which the rate of precipitation on air cooling is inappreciable, which comprises slow cooling said alloy at a rate not substantially greater than that of air cooling from austenite solution temperature through said transformation temperature, holding the alloy at a precipitation-hardening temperature for a time sufiicient to develop hardness therein, and cooling the alloy.

3. A method of precipitation-hardening a low carbon iron base alloy containing a minor proportion of aluminum as a precipitation-hardening constituent and of nickel and manganese, said alloy being capable of undergoing transformation from austenite to ferrite on cooling and having a content of nickel and manganese which is sufficient to lower the austenite-ferrite transformation temperature of the alloy to a level at which the rate of precipitation on air cooling is inappreciable, which comprises slow cooling said alloy at a rate not substantially greater than that of air cooling from austenite solution temperature through said transformation temperature, holding the alloy at a precipitation-hardening temperature for a time sufficient to develop hardness therein, and cooling the alloy.

4. A method of precipitation-hardening a low carbon iron base alloy which is capable of undergoing transformation from austenite to ferrite on cooling and contains about 0.5 to 5% aluminum and. about 0.2 to 20% nickel and manganese, the manganese being present in amount more than sufilcient to deoxidize the alloy and the excess manganese and the nickel together being efiective to lower the austenite-ferrite transformation temperature of the alloy to a level below 600 C., which comprises slow cooling said alloy at a rate not substantially greater than that of air cooling from austenite solution temperature through said transformation temperature, holding the alloy at a precipitation-hardening temperature for a time sufficient to develop hardness therein, and cooling the alloy.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,943,595 Foley Jan. 16, 1943 FOREIGN PATENTS Number Country Date 409,355 Great Britain Apr. 30, 1934 

1. A METHOD OF PRECIPITATION-HARDENING A LOW CARBON IRON BASE ALLOY CONTAINING A MINOR PROPORTION OF A PRECIPITATION-HARDENING CONSTITUENT AND OF A CONSTITUENT FROM THE GROUP CONSISTING OF NICKEL AND MANGANESE, SAID ALLOY BEING CAPABLE OF UNDERGOING TRANSFORMING FROM AUSTENITE TO FERRITE ON COOLING AND HAVING A CONSTITUENT CONTENT FROM SAID NICKEL AND MANGANESE GROUP WHICH IS SUFFICIENT TO LOWER THE AUSTENITE-FERRITE TRANSFORMATION TEMPERATURE OF THE ALLOY TO A LEVEL AT WHICH THE RATE OF PRECIPITATION ON AIR COOLING IS INAPPRECIABLE, WHICH COMPRISES SLOW COOLING SAID ALLOY AT A RATE NOT SUBSTANTIALLY GREATER THAN THAT OF AIR COOLING FROM AUSTENITE SOLUTION TEMPERATURE THROUGH SAID TRANSFORMATION TEMPERATURE, HOLDING THE ALLOY AT A PRECIPITATION-HARDENING TEMPERATURE FOR A TIME SUFFICIENT TO DEVELOP HARDNESS THEREIN, AND COOLING THE ALLOY. 